darwin and modern science-第93节

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〃Le　seul　parti　a　prendre　est　donc　d'envisager　les　etres　organises　comme　existant　depuis　certaines　epoques；　avec　leurs　qualites　particulieres。〃　　（Vol。　II。　page　1107。）

Reviewing　the　position　fourteen　years　afterwards；　Bentham　remarked：〃These　views；　generally　received　by　the　great　majority　of　naturalists　at　the　time　De　Candolle　wrote；　and　still　maintained　by　a　few；　must；　if　adhered　to；　check　all　further　enquiry　into　any　connection　of　facts　with　causes；〃　and　he　added；　〃there　is　little　doubt　but　that　if　De　Candolle　were　to　revise　his　work；　he　would　follow　the　example　of　so　many　other　eminent　naturalists；　and。。。insist　that　the　present　geographical　distribution　of　plants　was　in　most　instances　a　derivative　one；　altered　from　a　very　different　former　distribution。〃　　（〃Pres。　Addr。〃　（1869）　〃Proc。　Linn。　Soc。〃　1868…69；　page　lxviii。）

Writing　to　Asa　Gray　in　1856；　Darwin　gave　a　brief　preliminary　account　of　his　ideas　as　to　the　origin　of　species；　and　said　that　geographical　distribution　must　be　one　of　the　tests　of　their　validity。　　（〃Life　and　Letters〃；　II。　page　78。）　　What　is　of　supreme　interest　is　that　it　was　also　their　starting…point。　　He　tells　us：〃When　I　visited；　during　the　voyage　of　H。M。S。　〃Beagle〃；　the　Galapagos　Archipelago；。。。I　fancied　myself　brought　near　to　the　very　act　of　creation。　　I　often　asked　myself　how　these　many　peculiar　animals　and　plants　had　been　produced：　　the　simplest　answer　seemed　to　be　that　the　inhabitants　of　the　several　islands　had　descended　from　each　other；　undergoing　modification　in　the　course　of　their　descent。〃　　（〃The　Variation　of　Animals　and　Plants〃　（2nd　edition）；　1890；　I。　pages　9；　10。）　　We　need　not　be　surprised　then；　that　in　writing　in　1845　to　Sir　Joseph　Hooker；　he　speaks　of　〃that　grand　subject；　that　almost　keystone　of　the　laws　of　creation；　Geographical　Distribution。〃　　（〃Life　and　Letters〃；　I。　page　336。）

Yet　De　Candolle　was；　as　Bentham　saw；　unconsciously　feeling　his　way；　like　Lyell；　towards　evolution；　without　being　able　to　grasp　it。　　They　both　strove　to　explain　phenomena　by　means　of　agencies　which　they　saw　actually　at　work。　　If　De　Candolle　gave　up　the　ultimate　problem　as　insoluble：〃La　creation　ou　premiere　formation　des　etres　organises　echappe；　par　sa　nature　et　par　son　anciennete；　a　nos　moyens　d'observation〃　（Loc。　cit。　page　1106。）；　he　steadily　endeavoured　to　minimise　its　scope。　　At　least　half　of　his　great　work　is　devoted　to　the　researches　by　which　he　extricated　himself　from　a　belief　in　species　having　had　a　multiple　origin；　the　view　which　had　been　held　by　successive　naturalists　from　Gmelin　to　Agassiz。　　To　account　for　the　obvious　fact　that　species　constantly　occupy　dissevered　areas；　De　Candolle　made　a　minute　study　of　their　means　of　transport。　　This　was　found　to　dispose　of　the　vast　majority　of　cases；　and　the　remainder　he　accounted　for　by　geographical　change。　　（Loc。　cit。　page　1116。）

But　Darwin　strenuously　objected　to　invoking　geographical　change　as　a　solution　of　every　difficulty。　　He　had　apparently　long　satisfied　himself　as　to　the　〃permanence　of　continents　and　great　oceans。〃　　Dana；　he　tells　us　〃was；　I　believe；　the　first　man　who　maintained〃　this　（〃Life　and　Letters〃；　III。　page　247。　　Dana　says：〃The　continents　and　oceans　had　their　general　outline　or　form　defined　in　earliest　time；〃　〃Manual　of　Geology〃；　revised　edition。　　Philadelphia；　1869；　page　732。　　I　have　no　access　to　an　earlier　edition。）；　but　he　had　himself　probably　arrived　at　it　independently。　　Modern　physical　research　tends　to　confirm　it。　　The　earth's　centre　of　gravity；　as　pointed　out　by　Pratt　from　the　existence　of　the　Pacific　Ocean；　does　not　coincide　with　its　centre　of　figure；　and　it　has　been　conjectured　that　the　Pacific　Ocean　dates　its　origin　from　the　separation　of　the　moon　from　the　earth。

The　conjecture　appears　to　be　unnecessary。　　Love　shows　that　〃the　force　that　keeps　the　Pacific　Ocean　on　one　side　of　the　earth　is　gravity；　directed　more　towards　the　centre　of　gravity　than　the　centre　of　the　figure。〃　　（〃Report　of　the　77th　Meeting　of　the　British　Association〃　（Leicester；　1907）；　London；　1908；　page　431。）　　I　can　only　summarise　the　conclusions　of　a　technical　but　masterly　discussion。　　〃The　broad　general　features　of　the　distribution　of　continent　and　ocean　can　be　regarded　as　the　consequences　of　simple　causes　of　a　dynamical　character；〃　and　finally；　〃As　regards　the　contour　of　the　great　ocean　basins；　we　seem　to　be　justified　in　saying　that　the　earth　is　approximately　an　oblate　spheroid；　but　more　nearly　an　ellipsoid　with　three　unequal　axes；　having　its　surface　furrowed　according　to　the　formula　for　a　certain　spherical　harmonic　of　the　third　degree〃　（Ibid。　page　436。）；　and　he　shows　that　this　furrowed　surface　must　be　produced　〃if　the　density　is　greater　in　one　hemispheroid　than　in　the　other；　so　that　the　position　of　the　centre　of　gravity　is　eccentric。〃　　（Ibid。　page　431。）　　Such　a　modelling　of　the　earth's　surface　can　only　be　referred　to　a　primitive　period　of　plasticity。　　If　the　furrows　account　for　the　great　ocean　basins；　the　disposition　of　the　continents　seems　equally　to　follow。　　Sir　George　Darwin　has　pointed　out　that　they　necessarily　〃arise　from　a　supposed　primitive　viscosity　or　plasticity　of　the　earth's　mass。　　For　during　this　course　of　evolution　the　earth's　mass　must　have　suffered　a　screwing　motion；　so　that　the　polar　regions　have　travelled　a　little　from　west　to　east　relatively　to　the　equator。　　This　affords　a　possible　explanation　of　the　north　and　south　trend　of　our　great　continents。〃　　（〃Encycl。　Brit。〃　（9th　edition）；　Vol。　XXIII。　〃Tides〃；　page　379。）

It　would　be　trespassing　on　the　province　of　the　geologist　to　pursue　the　subject　at　any　length。　　But　as　Wallace　（〃Island　Life〃　（2nd　edition）；　1895；　page　103。）；　who　has　admirably　vindicated　Darwin's　position；　points　out；　the　〃question　of　the　permanence　of　our　continents。。。lies　at　the　root　of　all　our　inquiries　into　the　great　changes　of　the　earth　and　its　inhabitants。〃　　But　he　proceeds：　　〃The　very　same　evidence　which　has　been　adduced　to　prove　the　GENERAL　stability　and　permanence　of　our　continental　areas　also　goes　to　prove　that　they　have　been　subjected　to　wonderful　and　repeated　changes　in　DETAIL。〃　　（Loc。　cit。　page　101。）　　Darwin　of　course　would　have　admitted　this；　for　with　a　happy　expression　he　insisted　to　Lyell　（1856）　that　〃the　skeletons；　at　least；　of　our　continents　are　ancient。〃　　（〃More　Letters〃；　II。　page　135。）　　It　is　impossible　not　to　admire　the　courage　and　tenacity　with　which　he　carried　on　the　conflict　single…handed。　　But　he　failed　to　convince　Lyell。　　For　we　still　find　him　maintaining　in　the　last　edition　of　the　〃Principles〃：　　〃Continents　therefore；　although　permanent　for　whole　geological　epochs；　shift　their　positions　entirely　in　the　course　of　ages。〃　　（Lyell's　〃Principles　of　Geology〃　（11th　edition）；　London；　1872；　I。　page　258。）

Evidence；　however；　steadily　accumulates　in　Darwin's　support。　　His　position　still　remains　inexpugnable　that　it　is　not　permissible　to　invoke　geographical　change　to　explain　difficulties　in　distribution　without　valid　geological　and　physical　support。　　Writing　to　Mellard　Reade；　who　in　1878　had　said；　〃While　believing　that　the　ocean…depths　are　of　enormous　age；　it　is　impossible　to　reject　other　evidences　that　they　have　once　been　land；〃　he　pointed　out　〃the　statement　from　the　'Challenger'　that　all　sediment　is　deposited　within　one　or　two　hundred　miles　from　the　shores。〃　　（〃More　Letters〃；　II。　page　146。）　　The　following　year　Sir　Archibald　Geikie　（〃Geographical　Evolution〃；　〃Proc。　R。　Geogr。　Soc。〃　1879；　page　427。）　informed　the　Royal　Geographical　Society　that　〃No　part　of　the　results　obtained　by　the　'Challenger'　expedition　has　a　profounder　interest　for　geologists　and　geographers　than　the　proof　which　they　furnish　that　the　floor　of　the　ocean　basins　has　no　real　analogy　among　the　sedimentary　formations　which　form　most　of　the　framework　of　the　land。〃

Nor　has　Darwin's　earlier　argument　ever　been　upset。　　〃The　fact　which　I　pointed　out　many　years　ago；　that　all　oceanic　islands　are　volcanic　（except　St　Paul's；　and　now　that　is　viewed　by　some　as　the　nucleus　of　an　ancient　volcano）；　seem　to　me　a　strong　argument　that　no　continent　ever　occupied　the　great　oceans。〃　　（〃More　Letters〃；　II。　page　146。）

Dr　Guppy；　who　devoted　several　years　to　geological　and　botanical　investigations　in　the　Pacific；　found　himself　forced　to　similar　conclusions。　〃It　may　be　at　once　observed；〃　he　says；　〃that　my　belief　in　the　general　principle　that　islands　have　always　been　islands　has　not　been　shaken；〃　and　he　entirely　rejects　〃the　hypothesis　of　a　Pacific　continent。〃　　He　comes　back；　in　full　view　of　the　problems　on　the　spot；　to　the　position　from　which；　as　has　been　seen；　Darwin　started：　　〃If　the　distribution　of　a　particular　group　of　plants　or　animals　does　not　seem　to　accord　with　the　present　arrangement　of　the　land；　it　is　by　far　the　safest　plan；　even　after　exhausting　all　likely　modes　of　explanation；　not　to　invoke　the　intervention　of　geographical　changes；　and　I　scarcely　think　that　our　knowledge　of　any　one　group　of　organisms　is　ever　sufficiently　precise　to　justify　a　recourse　to　hypothetical　alterations　in　the　present　relations　of　land　and　sea。〃　　（〃Observations　of　a　Naturalist　in　the　Pacific　between　1896　and　1899〃；　London；　1903；　I。　page　380。）　　Wallace　clinches　the　matter　when　he　finds　〃almost　the　whole　of　the　vast　areas　of　the　Atlantic；　Pacific；　Indian；　and　Southern　Oceans；　without　a　solitary　relic　of　the　g